TRAINING COURSE ON RADIATION
DOSIMETRY:
Instrumentation 1 –
Gas detectors / Part 1
Anthony WAKER,
University of Ontario Instutute of Technology
Wed. 21/11/2012, 15:00 – 16:00 pm, and 16:30 – 17:30 pm
GAS-FILLED DETECTORS
• One of the oldest and most widely used radiation
detectors
• Gas-filled detectors sense the direct ionization
created by the passage of charged particles caused
by the interaction of the radiation with the chamber
gas
 Ion Chambers
 Proportional Counters
 Geiger-Mueller Counters
BASIC COMPONENTS OF AN IONIZATION CHAMBER
Common Fill Gases: Ar, He, H2, N2, Air, O2, CH4, TE
IONIZATION IN GASES
To create an ion pair, a minimum energy equal to the
ionization energy of the gas molecule must be
transferred
Ionization energy between 10 to 25 eV for least tightly
bound electron shells for gases of interest
 Competing mechanisms such as excitation leads to incident
particle energy loss without the creation of ion pair
W-value: average energy lost by incident particle per
ion pair formed
Typical W-values are in the range of 25 – 35 eV/ion pair
CHARGE COLLECTION
Under steady-state irradiation, rate of ion-pair
formation is constant
 For a small test volume, rate of formation will be exactly
balanced by rate at which ion pairs are lost from volume
due to recombination, diffusion or migration from the volume.
CHARGE COLLECTION
For ions in a gas,
𝒗𝒅𝒓𝒊𝒇𝒕
𝝁𝓔
=
𝒑
𝒗𝒅𝒓𝒊𝒇𝒕 − 𝒅𝒓𝒊𝒇𝒕 𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚 𝒐𝒇 𝒊𝒐𝒏𝒔
𝝁 − 𝒎𝒐𝒃𝒊𝒍𝒊𝒕𝒚
𝓔 − 𝒆𝒍𝒆𝒄𝒕𝒓𝒊𝒄 𝒇𝒊𝒆𝒍𝒅 𝒔𝒕𝒓𝒆𝒏𝒈𝒕𝒉
𝒑 − 𝒈𝒂𝒔 𝒑𝒓𝒆𝒔𝒔𝒖𝒓𝒆
+ve and –ve charges are swept towards their respective electrodes
with 𝑣𝑑𝑟𝑖𝑓𝑡
By increasing 𝑣𝑑𝑟𝑖𝑓𝑡 concentration of ions within the gas volume
decreases suppressing volume recombination within the gas volume.
RECOMBINATION
Two types of recombination:
Columnar (or initial) recombination
Increases with LET of Radiation
Volume recombination
Increases with dose-rate
𝑑𝑛+ 𝑑𝑛−
+
= −𝛼𝑛+ 𝑛−
𝑑𝑡
𝑑𝑡
CHARGE COLLECTION
IONIZATION CHAMBER – FARMER CHAMBER
IONIZATION CHAMBERS
What ionization current would we expect to
measure in a Farmer chamber placed in a high
energy photon radiotherapy beam where the dose
rate to air is 10 Gy per minute.
INSULATORS AND GUARD RINGS
Typical ionization currents are extremely small.
Require good insulation and guard rings to ensure
leakage current does not interfere with ionization
current
IONIZATION CHAMBER DOSIMETRY
DOSIMETRY WITH IONIZATION CHAMBERS
𝐷𝑚𝑎𝑡𝑡𝑒𝑟
𝑆
= 𝐷𝑐𝑎𝑣𝑖𝑡𝑦 .
𝜌
𝑚𝑎𝑡𝑡𝑒𝑟
𝑐𝑎𝑣𝑖𝑡𝑦
FANO’S THEOREM
In an infinite medium of given atomic composition exposed to a
uniform field of indirectly ionizing radiation, the field of secondary
radiation is also uniform and independent of density of the
medium, as well as density variations from point to point.
This means that if an ionization chamber is constructed of a
wall material and filled with gas of the same atomic
composition the dose to the wall material will be the same as
the dose measured to the gas regardless of the size of the
chamber
IONIZATION CHAMBER DOSIMETRY - CALIBRATION
IONIZATION CHAMBER DOSIMETRY
ATOMIC COMPOSITION OF TISSUE AND TE GAS
• Methane based
• CH4 (64.4% partial pressure)
• CO2 (32.4% partial pressure)
• N2 (3.2% partial pressure)
• By %weight: H (10.2); C (45.6); N (3.5); O
(40.7)
• Propane based
• C3H8 (% partial pressure)
• CO2 (% partial pressure)
• N2 (%partial pressure)
• By %weight: H (10.3); C (56.9); N (3.5); O
(29.3)
ICRU Tissue (Muscle) atomic
composition by % weight
H
C
N
O
10.2
12.3
3.5
72.9
TISSUE EQUIVALENT PLASTIC
The main tissue equivalent
plastic used in dosimetry is
A150.
The atomic composition of
A150 is close to tissue but
has a higher percentage by
weight of carbon, which
makes it conductive.
A150 TE-plastic atomic composition by % weight
H
C
N
O
muscle
(10.2)
muscle
(12.3)
muscle
(3.5)
muscle
(72.9)
10.1
77.6
3.5
5.2
GAS-GAIN IN PROPORTIONAL COUNTERS
A proportional counter is a gas-ionization device
consisting of a cathode, thin anode wire and fill-gas.
Ionization in the fill gas is multiplied providing an
amplified signal proportional to the original amount of
ionization.
GAS GAIN
The gas-gain achievable in a
proportional counter is determined
by the first Townsend coefficient α
for the counter fill gas used
α itself depends on the reduced
electric field in the counter, which is
determined by the applied voltage
and counter geometry
ln( G )    d
GAS GAIN
8
7
6
5
ln G*
To a first
approximation the
relationship
between the
logarithm of gasgain and applied
anode voltage is
linear
Relative Gas Gain for Propane Based TE Gas at Pressures 3.25
(graph 1), 6.5 (2), 16.25 (3), 26 (4), 32.5 torr, Relative to the
Measurement Made at 32.5 Torr and Vanode 100 V
Series1
4
Series2
3
Series3
Series4
2
Series5
1
0
0
100
200
300
400
V anode (V)
500
600
700
800
SIMULATION USING A GAS CAVITY
SITE-SIZE SIMULATION
Energy
deposited in the
gas cavity by a
charged particle
crossing the
cavity equals
energy
deposited in
tissue site by an
identical
particle
Eg
Et
SITE-SIZE SIMULATION
𝐸𝑔 = 𝐸𝑡
1 𝑑𝐸
1 𝑑𝐸
.
. 𝜌𝑔 . ∆ 𝑔 =
.
. 𝜌𝑡 . ∆ 𝑡
𝜌 𝑑𝑥 gas
𝜌 𝑑𝑥 tissue
SITE-SIZE SIMULATION
The density of
the gas in the
cavity is
adjusted to
equal the ratio
of the tissue
site diameter to
the gas cavity
diameter
Diameter of
Tissue Site
 X t
 g  

X
g

Density of Gas

t


Diameter of Gas
Cavity
Density of Tissue Site
(1000 kg.m-3)
EXAMPLE
𝝆𝒈 = (𝟏𝟎−𝟔 /𝟏𝟎−𝟐 ). 𝝆𝒕
What is the density of
propane TE gas
required for a 1 cm
cavity to simulate a
tissue sphere of 1 µm.
𝝆𝒈 = 10-4 . 1000 kg.m-3
𝝆𝒈 = 0.1kg.m-3
EXAMPLE
𝝆𝒈 = 𝟏. 𝟕𝟗𝟖 𝒌𝒈. 𝒎−𝟑 𝒂𝒕 𝟏𝟎𝟎 𝒌𝑷𝒂 𝒂𝒏𝒅 𝟐𝟎 𝒐𝑪
𝝆𝑷𝑻
What pressure of
propane TE gas is
required for a density
of 0.1 kg.m-3
𝑷 𝑻𝒐
= 𝝆𝑷𝒐𝑻𝒐 .
.
𝑷𝒐 𝑻
At 20 oC:
𝟎. 𝟏 = 𝟏. 𝟕𝟗𝟖.
𝑷 𝟐𝟗𝟑
.
𝟏𝟎𝟎 𝟐𝟗𝟑
𝑷 = 𝟓. 𝟓𝟔𝟐 𝒌𝑷𝒂
(41.72 torr)
Rossi Counter
INTERNAL SOURCE CALIBRATION
In crossing the TEPC
an alpha particle will
lose an average
amount of energy
that can be
calculated using
range-energy data
INTERNAL SOURCE CALIBRATION
Each alpha
particle crossing
the counter
generates a pulse
height
proportional to
the energy
deposited; the
mean of this
distribution is
associated with
the mean energy
deposited in the
counter
EXAMPLE
Applied Voltage 750V; amplifier
gain 10
Using range-energy data
for propane based TE
gas for a 2 micron
simulated diameter and a
Cm-244 internal alpha
source of energy 5.8 MeV.
Mean energy lost 168.48 keV
Mean chord-length 1.33 µm
Channel 3835 corresponds to
126.68 keV/µm
Calibration Factor for amplifier
setting of 10 (126.68/3835) =
0.03303 keV/µm/channel
Frequency distribution measured in an Am-Be
field with a 2” REM500 TEPC with simulated
diameter 2µm and calibration factor
1.641keV/µm/chn
Frequency x lineal energy: Dose
Distribution d(y) with calibration factor
1.641keV/µm/chn
y.f(y) data plotted in equal logarithmic intervals, 50 per decade
MEASUREABLE QUANTITIES – AMBIENT DOSE
EQUIVALENT
∗
∗
𝐻 (10) = 𝐷 (10). 𝑄
Estimated directly
from the measured
event-size spectrum
Determined from the
shape of the event-size
spectrum and assuming
Q(y) = Q(L)
TEPC MEASUREABLE QUANTITIES –
ABSORBED DOSE
From ICRP 60
Assuming that Lineal Energy y is equal to Linear Energy Transfer L
TEPC RESPONSE – KERMA
TEPC ENERGY RESPONSE – QUALITY FACTOR
GEMS
GAS ELECTRON MULTIPLIER
Operates as
proportional
counter except
multiplication
takes place
between the top
and bottom
surfaces of the
GEM structure
through
microscopically
etched holes
Dubeau and Waker Radiat. Prot. Dosim. 128(4), 2008
Dubeau and Waker Radiat. Prot. Dosim. 128(4), 2008
GEM SUMMARY
 GEMs can be configured to operate as TEPC and have the
advantage of
 smaller physical size for each detecting element and smaller
simulated diameters for improving dose equivalent response
(better LET spectrometers)
 Potential for basis as a personal neutron dosimeter
 Particle tracking capability depending on read-out pattern of
anode
 Much work still to be done!